Error importing several meshes from single dir
os.listdir only return name and extension and subdir for a Dir, you didn't indicate the relative directory path or absolute path for filepath to search the file by name.

You could use os.path.join() to join your path:

dir_path = ("C:\stlfiles")
files = os.listdir(dir_path) # if you just create a reference to dir, why not use it directly?

for file_name in files:
    if file_name.lower().endswith('.stl'):
        full_file_path = os.path.join(dir_path,file_name)
        pass #do something to full_file_path